The present invention relates generally to interconnecting analog and digital integrated circuits and, more particularly, to interconnecting such in a manner that reduces noise.
Analog electrical signals are often processed by digital computational circuitry. This requires converting the analog signal into a stream of discrete numerical values, in other words, a digital signal. This conversion typically takes place in an analog to digital (A/D) converter that periodically samples the analog signal and represents those samples with digital values. To convert digital signals back to analog signals requires a digital to analog (D/A) converter.
The digital circuits that process digital signals, however, generate noise that can interfere with the A/D and D/A conversion processes. This interference can lead to decreased accuracy or erroneous conversions. When low accuracy for such conversions will suffice, the converter circuits and the computational circuits can coexist on the same integrated circuit substrate. Where higher accuracy is required, however, the single substrate technique proves to be inadequate because of the noise produced by the digital circuits.
Currently, circuits for applications requiring high accuracy address this noise problem by the configuration in FIG. 1, which incorporates two separate integrated circuit devices, with separate substrates, into a single package. A substrate 10 contains analog circuitry 12, while a separate substrate 11 contains digital circuitry 13. The two substrates 10 and 11 are interconnected via traditional lead bonding wires 15. The lead bonding wires 15 are connected to substrates 10 and 11 through bonding pads 14. In the alternative, bonding wires 15 can be bonded from one substrate to a stationary lead frame element, and then from that element to the other substrate. These interconnections carry binary signals of the logic levels required by the logic circuit.
Unfortunately, this configuration does not eliminate interference from digital signals needed for conversion of signals. For D/A converters, data must be transferred from the digital circuit to the analog circuit, and for A/D converters, data must be transferred from the analog circuit to the digital circuit. Also, clock signals that drive the converters and synchronize the transfer of data between the analog and digital circuits are digital signals. These data and clock signals are carried by interconnect bonding pads on the analog circuit substrate. The bonding pads are capacitively coupled with the underlying substrate enough to allow the digital signals to create noise and interfere with the conversion process.
The bonding pad to substrate coupling problem is further aggravated by two factors. First, the advent of fine line lithography has made the conducting, insulating, and semiconducting features of the devices extremely small (fractions of a micron), while lead bonding technology continues to require comparatively large bonding pads (10,000 square microns in area). As this trend continues, the relative influence of bonding pad signals into the circuit substrate increases. Second, all circuits must have static protection circuits connected from the bonding pads to the substrate to protect against static discharge during packaging. These static protection circuits aggravate the bonding pad-to-substrate coupling problem.
In the particular case of a converter using switched capacitor, delta sigma conversion techniques, noise is also caused by the associated decimation filter. In that case, a very high clock frequency is used to sample both the analog input voltage and a reference voltage at the input of a switched capacitor filter. After each high frequency cycle, a comparator ascertains whether the filter output is positive or negative. The polarity of the reference potential on the next cycle is then determined by the output of the comparator. The converters output a single bit on each high frequency clock cycle and use a digital decimation filter to reduce the sample rate while increasing the resulting low sample frequency accuracy. A typical delta sigma converter capable of converting frequencies up to 20 kilohertz requires a 3.072 megahertz high frequency clock. The decimation filter, in turn, would output 16 bit samples at 48 kilohertz. For this typical case, the noise produced by the decimation filter can, if not properly isolated from the analog circuitry, reduce the accuracy of the sampled signals in the switched capacitor section, causing a loss of conversion accuracy.
In light of the foregoing, there is a need to interconnect signals between analog conversion devices and digital processing devices in a manner that reduces the noise coupled into the substrate of circuits.